1. Field of the Invention
The present invention relates to the use of cone beam volume computed tomography (CBVCT) in breast imaging and in particular to improvements therein that make better use of the radiation from the radiation source, the data output by the detector, or both. Throughout the present disclosure, it will be understood that references to breast imaging (mammography) are illustrative rather than limiting, as the invention is broadly applicable to breast imaging in general.
2. Description of Related Art
Breast cancer represents a significant health problem. More than 180,000 new cases are diagnosed, and nearly 45,000 women die of the disease each year in the United States.
The clinical goal of breast imaging is to detect tumor masses when they are as small as possible, preferably less than 10 mm in diameter. It is reported that women with mammographically detected, 1–10 mm invasive breast carcinoma have a 93% 16-year survival rate.
Conventional screen film mammography is the most effective tool for the early detection of breast cancer currently available. However, mammography has relatively low sensitivity to detect small breast cancers (under several millimeters). Specificity and the positive predictive value of mammography remain limited owing to an overlap in the appearances of benign and malignant lesions. Limited sensitivity and specificity in breast cancer detection of mammography are due to its poor contrast detectability, which is common for all types of projection imaging techniques (projection imaging can only have up to 10% contrast detectability). The sensitivity with which conventional mammography can identify malignant tumors in the pre-clinical phase will largely be affected by the nature of the surrounding breast parenchyma. Detection of calcifications will be influenced to a lesser degree by the surrounding tissue. The perception of breast masses without associated calcification, representing the majority of tumors in patients with detected carcinomas, is greatly influenced by the mammographic parenchymal pattern. Thus conventional mammography is often not able to directly detect tumors of a few millimeters due to poor low contrast resolution. Conventional mammography requires ultrahigh resolution (50–100 μm/pixel) to image microcalcifications to compensate for its poor contrast resolution. Mammography fails to initially demonstrate 30%–35% of cancers. In addition, not all breast cancers detected with mammography will be found early enough to cure. At best, it appears that conventional mammography can reduce the death rate by up to 50%. This is an important gain, but there is considerable room for improvement in early detection of breast cancer.
Relatively low specificity of mammography results in biopsy for indeterminate cases despite the disadvantages of higher cost and the stress it imposes on patients. There is a need for more accurate characterization of breast lesions in order to reduce the biopsy rate and false-positive rate of biopsy.
There are several radiological or biological characteristics of breast carcinoma that can be imaged. First, carcinoma has different x-ray linear attenuation coefficients from surrounding tissues, as shown in FIG. 1. Second, carcinoma has a substantially higher volume growth rate compared to a benign tumor which lacks growth. Third, carcinoma has patterns distinguishable from those of a benign tumor. Fourth, benign tumors show different contrast enhancement after intravenous contrast injection. Fifth, the presence of neovascularity can indicate cancer. Conventional mammography relies mainly on the first characteristic and partially uses the third characteristic for breast cancer detection. Since mammography is a two-dimensional static imaging technique, it cannot provide any information regarding characteristics 2, 4, or 5.
Currently, radiological evaluation of breast cancer is important not only for early detection of disease, but also for staging and monitoring response to treatment. So far, conventional screen film mammography has been shown to be the most cost-effective tool for the early detection of breast cancer currently available. The specificity and positive predictive value of mammography, however, remain limited, owing to an overlap in the appearances of benign and malignant lesions and to poor contrast detectability, which is common for all projection imaging techniques. Projection imaging can have only up to 10% contrast detectability. Biopsy is therefore often necessary in indeterminate cases, despite the disadvantages of higher cost and the stress it imposes on patients. There is therefore a need for more accurate characterization of breast lesions in order to reduce the biopsy rate.
In the last decade, MRI of the breast has gained a role in clarifying indeterminate cases after mammography and/or ultrasound, especially after breast surgery and in detecting multifocal breast cancers. However, the integration of MR into routine clinical practice has been hampered by a number of limitations, including long scanning times and the high cost of MR examinations. Additionally, many patients cannot undergo MR because of MR contraindications (e.g., aneurysm clips, pacemaker) or serious claustrophobia.
Characterization of breast lesions on MR has been based largely on the differential rates of enhancement between benign and malignant lesions. The constant trade-off between spatial and temporal resolution in MR has made it difficult to achieve the spatial resolution necessary for improved lesion detection and characterization.
Standard fan beam computed tomography (CT), including spiral CT, has been evaluated as a potential tool for the characterization of breast lesions. Most previous work has been based on the traditional or helical technique using the whole body scanner. That technique, however, suffers from a number of disadvantages including significantly increased radiation exposure due to the fact that standard CT can not be used to target only the breast, so that the majority of x-rays are wasted on whole body scanning. That leads to relatively low in-plane spatial resolution (typically 1.0 lp/mm), even lower through plane resolution (less than or equal to 0.5 lp/mm in the direction perpendicular to slices), and prolonged volume scanning times, since spiral CT scans the whole volume slice by slice and takes 120 seconds for the whole breast scan. It still takes 15–30 seconds for the latest multi-ring spiral CT for 1 mm/slice and 12 cm coverage.
Ultrasound has poor resolution in characterizing lesion margins and identifying microcalcifications. Ultrasound is also extremely operator dependent.
In addition, for conventional mammography, compression is essential for better low-contrast detectability. However, patients are uncomfortable even though compression may not be harmful to them.
The above-cited parent patent application teaches the use of cone beam volume computed tomography (CBVCT) for breast imaging, called cone beam volume CT breast imaging (CBVCTBI) technique. The patient lies on a table with a breast hole through which the breast to be imaged extends. A gantry frame rotates a radiation source and a detector around the breast. However, because of the relative positioning of the radiation source and the table required to achieve the volume scan, the radiation source may be positioned so low relative to the table that the entire breast may not lie within the cone beam (half cone beam). Also, it would be helpful to compensate for the shape of the breast while minimizing discomfort for the patient and to analyze efficiently the signals corresponding to various kinds of breast tissues and lesions. Furthermore, it would be helpful to increase sensitivity of detecting small breast cancer without raising patient dose levels or to reduce patient dose level without compromising image quality of CBVCTBI.